I continue to have success with the FNR and the API Nitra-Zorb in both the prep tank and the display tank. I would not do what your doing with Prime. Just use one of the nitrate remover products and recharge as necessary in warm salt water.

Edit: I just realized I didn't specifically answer your question about FNR in the prep tank. I'm using two batches of FNR in the AC70 filter I'm using on the 10g tank. I have been doing two batches (20g) then recharging the FNR in warm salt water. The FNR gets my off the chart nitrates down to 5ppm.
So far it's going strong. I theorize that because the resin is not seeing dissolved organics that plug the pores (as would be the case in a fish tank), the salt water continues to clean the resin of the trapped nitrates, making it ready for reuse (stored in brine until then).
I can't say that it will last forever, but it is going strong and is certainly a cost effective way of filtering out nitrates for water changes.
My garage is unheated so come winter I will need to relocate, most likely into the basement. At that time, I just may switch to using my old 29g that's down there.

As much as I thought this DIY denitrate filter had potential, I never did get anaerobic bacteria to handle nitrates. In addition, the Tom Aqua lifter pump would seem to have 'issues' that required me to disconnect the tube every other day and alternate between pumping air and water to get working properly.

It's disconnected now as I try a new experiment with deep sand and a new bacteria product. I'll report details and findings in a new thread.

mines been working for a while now. problem is if i want absolutly 0 nitrates comming out of it the flow has to be a very slow drip. im sure it helps but its nothing that makes any huge difference.

ive found using a pouch of purigen works awesome. i still do 50% w.c. every week or so as i always did but since the purigen my nitrates went from 80 or so after a week to maybe 20 if that.

i would think that constructing a small canister out of pvc like i did for my other filters and filling it with purigen would work amazingly well. tha being said purigen is not cheap but at least its rechargeable.

I have taken one of my AquaClear 70 filters and replaced the impeller with an AC20 impeller, significantly reducing the flow. I have it set for max re-filtration so there's only a trickle flow through. The chamber is filled with the Seachem Matrix/De*Nitrate mix to serve as a bio-filter. I use a fluval sponge pre-filter over the strainer on the input tube.
My other AC70 is mechanical only with sponge and fiber and I routinely replace/exchange material to force bio-filtration in the bio-filter.

ive found using a pouch of purigen works awesome. i still do 50% w.c. every week or so as i always did but since the purigen my nitrates went from 80 or so after a week to maybe 20 if that. .

Purigen is a good product that adsorbs dissolved organics. However, although I've used Purigen in the past, I do have some reservations using it when culturing (or attempting to culture) anaerobic bacteria for nitrates. It seems to me that these bacteria require nutrients that they would obtain from dissolved organics in the water column. I worry that the Purigen might remove the nitrogenous waste that is the sustenance for the bacteria. Of course, one could argue balance and if there are fewer nitrates, fewer anaerobic bacteria to process same is of no consequence.
I may need to ponder this further!

honestly if the nitrates are low im happy. if the easiest most effective way of achieving this is say a 4"x1' pcv pipe filled with purigen then so be it. im sure ill try this at some point as i get these ideas and then have to see of they work. i could have easily bought a TON of purigen and made one of these. instead of using 5 liters of de-nitrate and 1 1/3 liters of matrix.

it is working kinda but basicly the flow is so slow i almost consider it a failure. possibly after some time there will be enough bacteria in there and i will be able to crank up the flow.

on a side note i just ordered a t-5 setup off ebay hopefully its not a piece of crap. my plants dont ever grow with a new 6500k bulb. they just sustain for a while then degrade. i figure this should make a big difference.

i could have easily bought a TON of purigen and made one of these. instead of using 5 liters of de-nitrate and 1 1/3 liters of matrix.
it is working kinda but basicly the flow is so slow i almost consider it a failure. possibly after some time there will be enough bacteria in there and i will be able to crank up the flow.

What have you used for a bio-seed? I tried Stability (with the Matrix/De*Nitrate mix) but didn't seem get anywhere. Now that I've switched to the AC70 with the AC20 impeller, I'm testing Aquabella right now.
I've also migrated to deep [pool filter] sand.

Another consideration is nutrients. The Aquaripure uses 5ml of vodka or grenadine syrup once or twice a week to feed/stimulate the bacteria. I may try a few drops of grenadine every few days in the bio-filter just to see what happens.

I think any effort to reduce tank nitrates seems to be a multi-pronged effort. Plants, reduced/controlled feeding, tank/filter cleaning/maintenance, water changes....? along with de-nitrification.

Footnote: just to underscore the 'problem', I have very high nitrates in my well water. This is most likely due to the 95 acre farmers field across the road that gets ample amounts of organic and chemical fertilizers.
Since I need special water treatment/handling for water changes, minimizing tank nitrates and volume of water for changes is important to me.